In manufacturing an LCD (liquid crystal display) panel, the method of aligning LCD material so far has been as follows. As shown in FIG. 1, orientation material is printed in a thickness of 500-1000 .ANG. on a glass substrate 2, thereby forming a polymer alignment film 4. Then alignment film 3 is rubbed by a roller 5 on which a nylon or rayon cloth 6 is wound, and which revolves at a high speed. Thus alignment film 3 is oriented, and this is called `rubbing process`.
As shown in FIG. 1, in the case of the rubbing process, the LC molecules are arranged with a certain inclination angle relative to the surface of the alignment material. Because of the simplicity of the rubbing process, it is used in most of the mass productions.
However, the rubbing process can induce mechanical scratches and electrostatic charge, with the result that the thin film transistor is damaged and the yield is aggravated. Therefore recently, studies are carried out on a non-contact aligning method.
Meanwhile, recently, the liquid crystal display has become of a large scale, and the application of the liquid crystal display is being expanded to the wall-suspending TV and the notebook computers. In accordance with this trend, a high quality picture, a high prestige and a wide viewing angle are demanded.
In order to achieve the wide viewing angle, one can divide a picture cell into a plurality of sub-picture cells. This is the multi-domain technique and one of the easiest way to expand viewing angle. In the case where a multi-domain display is to be manufactured by using the alignment material which is originally intended for the rubbing process, there has to be carried out a complicated lithographic process which includes the coating of an alignment film, a rubbing, the coating of a photoresist, an exposure, a development, a rubbing and the removal of the photoresist.
A photo-polymerization type alignment-material can simplify these complicated process as shown in FIG. 2. A typical example of this non-rubbing process was described by M. Schadt et al (Jpn. J. Appl. Phys., Vol. 31, 1992, 2155), by Dae S. Kang et al (U.S. Pat. No. 5,464,669), and by Yuriy Reznikov (Jpn. J. Appl. Phys., Vo. 34, 1992, L1000). The photo-alignment is done in the following manner. That is, a photo-sensitive group which is coupled to the polymer makes a photo-response to ultra-violet rays which have been polarized by a polarizer 7. In this process, the alignment material is arrayed in order, with the result that ultimately the liquid crystal is oriented. Thus a photo-polymerization type LCD orientation film 9 is formed, and this method is called `photo-alignment`.
The photo-alignment materials which were described in patents and treatises are the ones which are formed by coupling cinnamate to polyvinyl alcohol. They are based on the principle that the divalent bond of cinnamate makes a cycloaddition reaction to the linearly polarized ultra-violet rays. Only the cinnamate group which corresponds to the polarization direction makes a cycloaddition reaction, and thus, the high molecules are arrayed in a certain direction so as to induce the alignment of the LC material. If such an alignment method is employed, domain-divided pixels can be obtained using only a mask, and therefore, the wide viewing angle can be realized in a simple manner.
However, in the conventional method, the photo-alignment material which uses the polyvinyl alcohol polymer as the main chain shows a low thermal stability. Further, the anchoring energy which is the degree of physical interaction between the orientation film and the LC is also low compared with the polyimide alignment material which resorts to the conventional rubbing process.
Therefore, the present inventors came to invent a polymaleimide or polyimide type photo-alignment material in which the conventional problems can be overcome, and in which the heat resistant property and the optical properties are superior.